1. Field of the Invention
The present invention relates generally to the treatment of pain. More particularly, it relates to the treatment of pain through electrical nerve stimulation.
2. Description of Related Art
Neuropathic pain is a potentially debilitating symptom arising from a wide variety of causes including diabetes, amputation, various malignant and benign cancers, certain infectious diseases and hereditary conditions. This pain is often progressive and generally due to direct cellular damage or impingement of peripheral nerve tissues. It is typically treated with non-steroidal anti-inflammatory drugs, opioids, anticonvulsants and antidepressants. However, many cases are not well controlled on any of these medications. Even for cases where these drugs are effective, finding the correct drug and dose is often a long and intensive procedure of trial and error.
Neuropathic pain is an extremely common disorder. According to a French study [1] it is estimated that 6.9% of the population suffers from neuropathic pain and in 5.1% of the population the pain is moderate to severe. A separate British study [2] estimated the size of the affected population closer to 8%. It is further estimated that 10% to 15% of these patients do not respond to conventional treatments [3]. Alternative treatments that inhibit or prevent the transmission of pain signals through the nerves that carry them would be desirable.
Nerves are made up of bundles of nerve fibers called fascicles, which are bound by perineurium membranes. Each fascicle contains a large number of fibers of different sizes (diameters) and having different functions (motor, sensory, pain-transmitting, etc.). Some of the fibers within the fascicles have a myelin sheath coating the fiber, which produces faster signal-transmission rates through the fiber. Other fibers do not have a myelin sheath, resulting in slower signal-transmission rates. Larger, myelinated fibers (for example Aα- and Aβ-fibers) having diameters on the order of 10 μm typically are responsible for sensory and motor-control functions. Conversely, pain signals, such as those responsible for neuropathic pain, are transmitted via much smaller (diameters less than 1 μm), e.g. unmyelinated C-fibers, which can have diameters on the order of 0.1 μm or lower. To treat pain, it is desirable to inhibit the transmission of signals through the unmyelinated small fiber strands (such as C-fibers), while not substantially affecting the transmission characteristics of (or activating) the larger fibers responsible for motor control and other sensory functions.
Electrical stimulation of nerves carrying pain signals has been used to attempt to attenuate the sensation of pain. These include spinal cord stimulation (SCS), transcutaneous electrical stimulation (TENS), percutaneous electrical stimulation (PENS), and Peripheral Nerve Field Stimulation (PNFS). These techniques are thought to operate by inducing parasthesia, or a pins and needles sensation, which overwhelms much of the pain response but is not thought to inhibit transmission of the pain signals. The sensation of parasthesia simply masks the pain so it is less evident to the patient. But this effect can be reduced over time, as the patient becomes acclimated to the parasthesia and begins to notice once again the underlying pain. In addition, parasthesia itself can be unpleasant, and in some patients may actually exacerbate the perception of pain.
Current literature on blocking electrical nerve signals does not describe blocking signal-transmission through the smaller, unmyelinated C-fibers, but instead focuses on larger Aα and Aβ motor units. Moreover, the aforementioned techniques often cannot be focused on just the pain-signal carrying fibers, but instead also affect larger nerve fibers. As a result, using current techniques to induce parasthesia to mask the pain signals transmitted through small, unmyelinated fibers can undesirably affect the transmission characteristics of larger fibers responsible for higher sensory and motor functions, and even activate those fibers causing involuntary muscle contractions (spasm) and other sensory side effects. It is desirable to inhibit, e.g. block, the transmission of pain signals through the small, unmyelinated fibers that principally carry them. Ideally, this will be achieved without substantially affecting the larger fibers responsible for higher sensory and motor functions.